Interleukin-7 (IL-7), a cytokine that plays a key role in the generation, activation and homeostasis of the T-cell compartment, is currently under clinical investigation as a potential therapeutic agent for the immune reconstitution of HIV-1-infected patients. The role of IL-7 in the course of HIV-1 infection is still uncertain. Increased plasma levels of IL-7 have been documented in the course of HIV infection, in association with a reduced expression of the ?-chain of the IL-7 receptor (CD127) in both CD4+ and CD8+ T cells. The increase in plasma IL-7 was found to correlate with reduced peripheral CD4+ T-cell counts and higher levels of plasma viral load. The most plausible interpretation of these findings is a compensatory effort by the immune system to restore homeostatic levels of CD4+ T cells. However, it is evident that in patients with advanced HIV-1 disease this compensatory effort is insufficient. Thus, it has been suggested that the early administration of IL-7 to patients chronically infected with HIV-1 may help to limit the depletion of CD4+ T cells and/or to promote the regeneration of naive T cells from the precursor pool. Experimental injection of IL-7 in vivo in macaques chronically infected with SIV was shown to induce an increase in naive T-cell counts, as well as proliferation and activation of both CD4+ and CD8+ memory T cells, without augmenting the viral load. A phase I, randomized, placebo-controlled, double-blind trial (IND # BB12069) sponsored by NIAID is currently under way for evaluating the safety of subcutaneous single dose IL-7 in HIV-1-infected subjects who are receiving antiretroviral treatment. [unreadable] Since IL-7 is known to exert antiapoptotic effects during the course of T-cell development, we have investigated its ability to protect T cells from spontaneous apoptosis in a cohort of HIV-1-infected subjects selected to represent different stages of disease. Recombinant human IL-7 was used over a wide dose range (0.5-50 ng/ml) in an attempt to determine whether its prosurvival and proliferative activities can be uncoupled. In accordance with previous results, we found levels of spontaneous apoptosis at baseline were similar in cells from HIV-1-infected patients and HIV-seronegative controls, although they became significantly higher in cells from HIV-1-infected subjects during the course of 6 days of ex vivo culture. Upon addition of exogenous IL-7, a dramatic reduction in the level of spontaneous apoptosis was observed in peripheral blood mononuclear cells (PBMC) from all the HIV-1-infected patients tested (n = 24), as determined by two independent methods (annexin V binding and caspase 3 activation). The reduction of apoptosis mediated by IL-7 was associated with an increase in the intracellular level of Bcl-2. By contrast, IL-7 had a less consistent, not significant, antiapoptotic effect on PBMC from HIV-1-seronegative subjects. To gain further insights into the in vivo significance of IL-7-mediated apoptosis reduction, we analysed the correlation between the sensitivity to IL-7 and several demographic, clinical and immunological parameters, including age, disease duration, CD4 and CD8 counts and treatment status. A significant inverse correlation was documented between the sensitivity to the antiapoptotic effect of IL-7 and the circulating CD4 counts, while all the other parameters were not correlated. This correlation was confirmed by comparison of the IL-7 effects in different groups of patients divided according to a defined cut-off value for each demographic clinical and immunological parameter. To verify whether the protective effects of IL-7 occurred on both CD4+ and CD8+ T cells, we purified the two subpopulations from the peripheral blood of HIV-1-infected patients and cultured them ex vivo in the presence or absence of IL-7. These studies demonstrated that IL-7 exerts a protective anti-apoptotic effect of similar magnitude on both T-cell subpopulations from HIV-1-infected subjects. [unreadable] A potential drawback with the in vivo use of immunostimulatory cytokines such as IL-2 and IL-7 is the risk of triggering widespread T-cell activation and proliferation, which could further increase the already high level of immunologic stimulation that occurs in HIV-1 infection and thereby provide more target cells for HIV-1 replication. Thus, we investigated whether the use of IL-7 at concentrations that are sufficient to protect from spontaneous apoptosis was associated with cellular activation and/or proliferation. Intracellular staining with an antibody to the Ki67 antigen, a specific marker of cell cycle progression, showed that IL-7-mediated protection from apoptosis was temporally dissociated from Ki67 expression. Likewise, using the CFSE vital dye or absolute cell counting by flow cytometry, we formally demonstrated that the anti-apoptotic effect of IL-7 occurred in the absence of cell division. Moreover, we measured the levels of endogenous HIV-1 p24 release in cultures of PBMC or purified CD4+ T cells from selected HIV-1-infected patients. We observed that treatment with IL-7, in the absence of mitogenic stimulation, did not result in the induction of significant levels of endogenous HIV-1 replication during short-term ex vivo culture (7-14 days). [unreadable] The antiapoptotic activity that we documented for IL-7 provides an additional rationale for consideration of this cytokine as an immunotherapeutic agent in the treatment of HIV-1 infection, particularly in patients with more advanced disease who are less likely to regain full immune reconstitution with HAART and whose cells appear to be more sensitive to the anti-apoptotic effects of IL-7.